In recent years, an electronic device is made smaller and smaller and becomes more sophisticated. A temperature change inside of a housing of the electronic device becomes relatively larger as a result of such a smaller-sized and more-sophisticated structure of the electronic device. Accordingly, expansion and/or contraction of a printed circuit board have become a problem. A stress generated by the expansion and/or the contraction of the printed circuit board is applied to soldering portions of an electronic part or component mounted to the printed circuit board. Then, reliability of electrical connection at the soldering portions may be decreased.
In most of the electronic devices installed in a vehicle, the printed circuit board is fixed to the housing by screw members. It has been investigated and proposed that a torque for screwing the screw member is reduced so as to decrease a fastening force of the screw member. Then, it becomes possible that the printed circuit board moves in a surface direction thereof and thereby the stress applied to the soldering portion is decreased. However, when the fastening force of the screw member becomes smaller, the screw member may be loosened by the stress generated by the expansion and/or contraction of the printed circuit board. In such a case, a noise may be generated due to a movement or vibration of the printed circuit board within the housing and/or heat radiating performance may be decreased due to a change of a relative position between the printed circuit board and the housing.
For example, Japanese Patent Publication No. H10-145060 discloses an electronic device, which has a housing, two printed circuit boards (a first and a second printed circuit boards), a first and a second connectors respectively mounted to the first and the second printed circuit boards and electrically connected to each other, multiple screws for fixing each of the printed circuit boards to the housing, and so on. Each of the connectors is soldered to the respective printed circuit boards. Multiple supporting pillars are formed in the housing, wherein a screw portion is formed at a forward end of each supporting pillar. A first printed circuit board has multiple through-holes so that each of the forward ends of the supporting pillars is inserted through the through-holes. A screw member is screwed into each threaded portion of the supporting pillar in order to hold the first printed circuit board in the housing. The first printed circuit board is held between screw heads and supporting wall portions of the housing, so that the first printed circuit board can move in its surface direction with respect to the housing. The first connector soldered to the first printed circuit board is electrically connected to the second connector soldered to the second printed circuit board. Since the first printed circuit board can move with respect to the second printed circuit board, the first connector of the first printed circuit board can be easily moved to a proper position with respect to the second connector of the second printed circuit board, when the first and the second connectors are electrically connected to each other. Accordingly, any stress to be applied to soldering portions of the first and the second connectors can be reduced.
In the electronic device of the above prior art, the first printed circuit board is elastically bent in a thickness direction of the printed circuit board by a fastening force of the screw member. The printed circuit board is held in the housing between the screw heads of the screw members and the supporting wall portions of the housing by a reaction force of the printed circuit board. Therefore, the printed circuit board can move in its surface direction not only during a manufacturing process (including a step for electrically connecting the first and the second connectors of the two printed circuit boards to each other) but also when the printed circuit board is expanded and/or contracted during its actual use.
However, as explained above, the printed circuit board is elastically bent in the thickness direction by the fastening force of the screw member. As a result, stress caused by a bending force for the printed circuit board may be applied to soldering portions for the connector and other electronic components. Reliability for electrical connection at the soldering portions may be decreased.